1. Field of the Invention
The present invention relates in general to an automatic transmission suited for use in an automotive vehicle and more particularly to a control system for the automatic transmission, which attenuates N - D
select shock.
2. Description of the Prior Art
NISSAN AUTOMATIC TRANSMISSION SERVICE MANUAL published in 1984 discloses a control system for a four forward speed type automatic transmission, which includes a low clutch and a fixed orifice which is disposed in a fluid line which fluidly connects a source of line pressure with the servo chamber of the low clutch. The control system further comprises an accumulator which is operatively connected to the fluid line in a manner to modify the rate at which hydraulic pressure develops in the clutch servo chamber and smooth the engagement characteristics of the clutch during shifting.
That is, as is seen from the graph of FIG. 1, by the function of the fixed orifice, the raising rate of hydraulic pressure P.sub.L/C in the low clutch servo chamber is reduced as is illustrated by a chain line, and due to the function of the accumulator, the hydraulic pressure P.sub.L/C in the servo chamber temporarily levels off at pressure Pa before the same increases to become equal to that of the line pressure.
However, due to its inherent construction, the system has the following drawback.
That is, when the transmission is shifted from neutral (N) to D range, a notable N-D select shock tends to occur. To overcome this, it is possible to reduce the size of the fixed orifice and increase the flow resistance provided thereby. However, this causes the charging time of the accumulator to become excessively long and leads to an unacceptably increased N-D time lag. This will be understood from the following.
That is, as is understood from the graph of FIG. 1, when the size of the orifice is reduced, the time t' (viz., the time which does not take part in the shift shock because during which the low clutch is under a loss-stroke) elapsing from the time t.sub.0 when N-D select operation takes place to the time when the hydraulic pressure P.sub.L/C becomes to assume the level-off value Pa is increased and thus the time T' at which operation of the low clutch is finished is delayed causing the unacceptably increased N-D time lag.